Laser printers, particularly flying spot printers, are used in a wide variety of applications, from office printing, to medical printing, to bar code printing. These systems typically print with a single lower power beam, onto fairly sensitive media. Other systems, such as those used in the graphic arts industry, often are configured as multispot printers to attain sufficient productivity. As many of the graphic arts media are rather insensitive to light exposure, each of these incident beams must provide a high light level in a small spot at the printing plane. The printer is typically configured like a "lathe", where the page scan is obtained by rotating a drum which holds the film, and line scan, by translating the multiple laser beams in a direction parallel to the axis of rotation of the drum.
There are several approaches to configuring a printhead so as to solve this problem. In one approach, each of the laser sources is separately coupled to optical fibers, which are then mounted to form a linear array of sources. Each of these channels can then be independently modulated. Such systems are described in U.S. Pat. Nos. 4,900,130 and 5,351,617. Another approach is to utilize a monolithic array of laser sources and then image the elements of the laser array directly onto the light sensitive media to produce multiple spots. Power to each element of the laser array is individually modulated to obtain pixel densities. Such a system, described by U.S. Pat. No. 4,804,975 is potentially of lower cost and higher efficiency as compared to systems which couple the lasers to optical fibers. However, these systems are significantly disadvantaged as the failure of even one lasing element will appear as an artifact in the printed image, requiring replacement of the entire laser array source.
One approach to improving a printing system using a monolithic laser diode array source is to split each lasing element into an array of subarray sources, such as described in U.S. Pat. No. 5,619,245. Each writing element is assembled from the combined light of all the lasing elements of a given subarray, and each of the subarrays are directly and individually modulated to provide the image data input. This approach desensitizes the system to the failure of the lasing elements within a subarray.
Yet another approach to improving the design of a printing system with a monolithic diode laser array source is to combine the light from each lasing element to flood illuminate a spatial light modulator. The elements of the modulator break up the light into image elements, and each element of the modulator is subsequently imaged onto the media plane to form the desired array of printing spots. Printing systems employing this approach are described in U.S. Pat. Nos. 4,786,918, 5,517,359, and 5,521,748. These systems improve upon the conventional designs by providing indirect light modulation means, so that the laser diode array operates at full power, and serves only as a light source. Also, as the light from the emitters typically illuminates the modulator in an overlapping fashion, the resulting redundancy desensitizes the system to the failure or poor behavior of any the lasing elements within the array.
One factor effecting the design of such systems is the degree of uniformity of the illumination provided to the modulator. The system of U.S. Pat. No. 4,786,918 essentially relies on far field averaging of single mode sources to improve the illumination profile. The systems of U.S. Pat. Nos. 5,517,359 and 5,521,748 image multi-mode emitting elements onto the modulator in overlapped fashion at high magnification, and are therefore sensitive to the macro- and micro-nonuniformities of the light emission profiles. U.S. Pat. No. 5,517,359 includes a mirror system which provides partial compensation by substantially removing the macro-nonuniformities, but at the cost of some reduced brightness. Another system, described in U.S. patent application Ser. No. 08/757,889, filed Nov. 27, 1996, and assigned to the same assignee as the present invention, provides a different approach for improving the illumination uniformity to the modulator plane, by means of a fly's eye integrator design.
The brightness of the illumination to the modulator and the media can also be very much a factor in the design of laser printers, such as the ones described in U.S. Pat. Nos. 5,517,359, and 5,521,748. Many new media have been developed recently, in response to environmental concerns, which are increasingly insensitive, thereby requiring high exposure levels, typically 0.2-0.5 joules/cm.sup.2 or greater. Generally, these new media are thermally responsive to the absorbed laser light, with a high threshold required to induce the desired effect. As a result, the illumination to the media must have both a high power level and a high power density. For example, a laser array such as the Opto-power OPC-D020 laser (Opto Power Corporation, Tucson Ariz.), which emits 20 Watts from 19 emitters, each 150 .mu.m wide, and spaced apart on a 650 .mu.m pitch, can be used effectively in such a system. As the multi-mode laser diode array sources become increasingly brighter, emitting more light from smaller areas, further gains can be made. Eventually, an array of high power single mode emitters may be the ultimate source, provided that spatial coherence interference effects are minimal.
As an alternative to yet higher brightness laser array sources, it would be advantageous in the design of such laser printer systems to combine multiple laser diode array sources in order to illuminate the modulator, and then the media, with light at a higher power level and a higher power density. For example, it is well known in the art to combine light from polarized sources by means of polarization sensitive devices, such as polarization beam splitter cubes. For highly polarized light sources, polarization combining can be very advantageous, as the two beams can be completely overlapped, spatially, and angularly, thereby effectively increasing the brightness of the light downstream of the combiner relative to that of one source. Of course, polarization combining is difficult to employ if the system is polarization sensitive. For example, in a laser printer using a polarization sensitive modulator such as a PLZT device, the modulated contrast would be reduced by providing other than linearly polarized illumination. Likewise, wavelength combining can be used in similar fashion when multiple sources are of different wavelengths. This can be naturally valuable in a system such as a laser printer which creates color prints. Otherwise, for a nominally monochromatic system, care must be taken concerning the wavelength sensitivity of downstream elements, such as the modulator, optics, or recording media. Furthermore, if the light sources are of nominally the same wavelength, then exploiting small differences for the purposes of combining may be difficult both in terms of the design of the combiner and in the cost of source selection.
As another alternative to polarization or wavelength combining, spatial and angular combining can be used, although while the available power is increased, brightness is not. In such systems, the beams for the multiple sources are spatially or angularly adjacent. For example, the spatially variant mirror array of U.S. Pat. No. 5,155,623, Miller et al., allows two arrays of beams to be combined by introducing the light from the second array in the spatial gaps of the first. In U.S. Pat. No. 5,519,432, a roof mirror is used to combine two beams with angular adjacency, such that they appear to originate from a substantially coincident virtual point source. It can be important in such systems to minimize any residual spatial or angular gaps when light from multiple sources is combined by such means, as the system lagrange invariant is increased, or the net brightness is decreased.